Stitching apparatuses of various designs and configurations have been devised to form lock-stitches in fabric. A series of lock-stitches can be arranged on fabric to form an embroidery pattern. These embroidery patterns are programmed into a computer or stitching control system which moves the needle and fabric to lock-stitch a desired embroidery pattern. A top thread and a bobbin thread cooperate with the needle to form the lock-stitch in the fabric. The top thread originates from an upper side of the fabric while the bobbin thread originates from a lower side of the fabric. Modern embroidery apparatuses often have multiple needles per head with multiple heads per machine, with some embroidery apparatuses having 12 needles for each of 30 heads. Commonly each needle stitches a different color thread and each head embroiders a different piece of fabric. The corresponding needles on each head typically have the same color thread. Only one needle per head is active at any one time so that each head stitches the same color thread at the same time. But to simplify this description, primarily a single-needle, single-head, machine is discussed herein.
A proper ratio between the top thread and bobbin thread length is generally desirable for high quality and attractive stitching. The thread length ratio is affected by a tension ratio between the top and bobbin threads. The tension ratio affects the amount of top thread and bobbin thread length used for a particular stitch. It is known to adjust the resistance of a tensioning wheel to change the top thread tension and to adjust a spring and set screw on bobbin case to change the bobbin tension. Resistance is provided by the tensioning wheel and bobbin spring which can be varied in proportion to thread tension. Generally, tension for the top and bobbin threads is set by an operator for a particular thread only once at the beginning of stitching. The top thread tension can later be adjusted at the tensioning assembly, while the bobbin tension cannot be readjusted without stopping the machine. By changing the top thread tension the thread tension ratio can be adjusted because the bobbin tension is typically held constant. Changes in the tension ratio are reflected in changes in the thread length ratio. Research indicates that different stitches in an embroidery pattern require different tension ratios, but the prior art only allows manual adjustments at infrequent intervals.
The operator must observe the quality of the stitches while a pattern is being stitched to determine if the thread tension ratio is set properly. If the ratio is improper, it means either the top and/or bobbin thread tension requires adjustment. To correct the tension, the operator must manually adjust the top thread tension. When the proper thread tension ratio cannot be achieved by merely adjusting the top thread tension, the tension on the bobbin thread would require adjusting which would require stopping the stitching apparatus. Training is generally required for the operator to recognize problems with the tension ratio and for the operator to adjust the thread tension.
To assure the best quality stitch possible with infrequent tension adjustments, the tension for both top and bobbin threads are typically set too high. Excessive tension often leads to thread breakage. Additionally, excessive tension causes columns to be too narrow. This distortion can expose walking stitches meant to be concealed underneath. On the other hand, simply lowering the tension at the beginning of the pattern would cause some stitches to receive too little tension which would produce unsatisfactory embroidery such as looping. A general need is recognized to lower tension when required by some stitches in a pattern to avoid thread breakage and distortion, without decreasing the tension for other stitches.
It is known in the prior art of related fields to automatically adjust the present thread tension based upon past thread usage. For example, when an excessive amount of thread for past stitches has been consumed, the tension is increased. This type of tension adjustment does not take into account any of the unique factors related to lock-stitching or anticipate changes required for future stitches. Therefore, there is a need for methods which automatically control thread tension when performing lock-stitching.
Although the prior art describes manually adjusting the thread tension ratio in stitching apparatuses, manual adjustment of the tension ratio is undesirable because it requires trained operators, costly additional labor and may require the machine to be stopped. The infrequent tension adjustments in the prior art is problematic because different stitches in the embroidery pattern require different tensions to ensure uniform quality.